Finger sensor including enhanced ESD protection and associated methods

ABSTRACT

A finger sensor may include a finger sensing integrated circuit (IC) having a finger sensing area and at least one bond pad adjacent thereto, and a flexible circuit coupled to the IC finger sensor. More particularly, the flexible circuit may include a flexible layer, and at least one conductive trace carried thereby and coupled to the at least one bond pad. The sensor may also include at least one Electrostatic Discharge (ESD) electrode carried by the flexible layer. The ESD electrode may be positioned adjacent a beveled edge, for example, of an IC carrier and thereby exposed through a small gap between an adjacent portion of a frame.

FIELD OF THE INVENTION

The present invention relates to the field of electronics, and, moreparticularly, to the field of finger sensors including finger sensingintegrated circuits, and associated manufacturing methods.

BACKGROUND OF THE INVENTION

Sensors including integrated circuits (ICs) that directly sense thephysical properties of objects in the sensor's environment have comeinto widespread use in electronic equipment. These ICs are desirably inclose proximity to the external environments they measure, but theyshould not be damaged by the mechanical and/or electrical events that anexternal environment can apply.

One type of such sensing is finger sensing and associated matching thathave become a reliable and widely used technique for personalidentification or verification. In particular, a common approach tofingerprint identification involves scanning a sample fingerprint or animage thereof and storing the image and/or unique characteristics of thefingerprint image. The characteristics of a sample fingerprint may becompared to information for reference fingerprints already in a databaseto determine proper identification of a person, such as for verificationpurposes.

A particularly advantageous approach to fingerprint sensing is disclosedin U.S. Pat. Nos. 5,963,679 and 6,259,804, assigned to the assignee ofthe present invention, the entire contents of which are incorporatedherein by reference. The fingerprint sensor is an integrated circuitsensor that drives the user's finger with an electric field signal andsenses the electric field with an array of electric field sensing pixelson the integrated circuit substrate. Additional finger sensingintegrated circuits and methods are disclosed in U.S. Published U.S.Patent Application No. 2005/0089202 entitled “Multi-biometric fingersensor including electric field sensing pixels and associated methods”,also assigned to the assignee of the present invention, and the entirecontents of which are incorporated herein by reference.

A number of prior art references disclose various types of packaging ofIC sensors. For example, U.S. Pat. No. 6,646,316 to Wu et al. disclosesan optical sensor including a sensing die with bond pads on an uppersurface thereof. A flexible circuit board is coupled to the bond pads,and has an opening over the sensing surface. A transparent glass layercovers the opening in the flexible circuit board. U.S. Pat. No.6,924,496 to Manansala discloses a similar flexible circuit attachmentto a fingerprint sensor, but leaves the area above the surface open.

U.S. Pat. No. 7,090,139 to Kasuga et al. discloses a smart cardincluding a fingerprint sensor having bond pads attached to wiring film,and also including a window or opening above the sensing surface. U.S.Published Patent Application No. 2005/0139685 to Kozlay discloses asimilar arrangement for a fingerprint sensor.

Some fingerprint sensors are based on thin film technology, such asdisclosed in U.S. Published Application No. 2006/0050935 A1 to Bustgenset al. Other fingerprint sensors may include sensing elements on aflexible substrate, such as disclosed in U.S. Pat. No. 7,099,496 toBenkley, III. These sensors may be slightly more rugged that integratedcircuit based sensors, but may have performance shortcomings.

U.S. Published Patent Application No. 2005/0031174 A1 to Ryhanen et al.discloses a flexible circuit board covering an ASIC for capacitiveelectrode fingerprint sensing, and wherein the sensing electrodes are onthe surface of the flexible substrate and covered with a thin protectivepolymer layer. In some embodiments, the sensor may wrap the flexiblecircuit around to the back side of the ASIC for attachment to a circuitboard in a ball grid form.

U.S. Pat. No. 5,887,343, assigned to the assignee of the presentinvention, discloses an embodiment of a fingerprint sensor package thatincludes a transparent layer over the finger sensing area of a fingersensing IC. A chip carrier, having an opening for the sensing area, iscoupled, either capacitively or electrically, to the bond pads on the ICvia peripheral regions of the transparent layer.

Finger sensing ICs are currently used on some cellular telephonehandsets to capture fingerprints for user identification and to capturefinger motions for menu navigation. Standard IC packaging methods thatcompletely enclose the silicon chip are not used with these sensorsbecause the sensing fields the sensors use to measure the fingerprint(e.g., electric fields, thermal fields, etc.) do not pass effectivelythrough the package. For these sensors in today's systems, the IC orchip is typically packaged such that the finger can directly contact thepassivation layer on the chip surface during the reading operation. Forprotection from physical damage during storage and transport (in apocket or purse) the handsets are typically designed to fold closed whennot in operation, protecting the sensor assembly which is mounted on aninside surface of the folding device.

There are many situations, however, where it may be preferable to beable to mount the sensor on an unprotected external surface of thehandset. This would allow the sensor to be used without opening theclamshell handset, and would allow IC sensors to be used on handsetsthat do not fold closed, such as the so-called “candy bar” phones.

Unfortunately, the use of a finger sensing IC exposed on a device'sexternal surface will likely subject the sensor to mechanical and/orelectrical stresses not seen by a sensor that has a folding cover overit during storage. For example, a device in a pocket or purse will besubject to scratching, abrasion, point impact, continuous pointpressure, and shear impact forces. The packaging technologies used forsensors in closeable cases are unlikely to provide adequate protectionfor the silicon chip.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to provide a finger sensor with enhanced packaging andESD features and related methods.

This and other objects, features and advantages in accordance with thepresent invention are provided by a finger sensor comprising a fingersensing integrated circuit (IC) including a finger sensing area and atleast one bond pad adjacent thereto, and a flexible circuit coupled tothe IC finger sensor. More particularly, the flexible circuit maycomprise a flexible layer, and at least one conductive trace carriedthereby and coupled to the at least one bond pad. In addition, thesensor may also include at least one Electrostatic Discharge (ESD)electrode carried by the flexible layer. Accordingly, a cost effectivepackage may be provided while additionally providing enhanced ESDprotection.

In some embodiments, the finger sensor may further comprise an ICcarrier having a cavity receiving the IC finger sensor therein, andwherein the IC carrier has at least one beveled edge adjacent the atleast one ESD electrode. In addition, a frame may surround at least aportion of an upper perimeter of the flexible layer and define therewithat least one ESD passage to the at least one ESD electrode. In otherwords, this packaging configuration will effectively drain off ESDthrough a small gap between the frame and flexible layer.

A fill material may be between the IC finger sensor and the flexiblecircuit. In addition, the flexible circuit may comprise at least oneconnector portion extending beyond the finger sensing area and theplurality of bond pads. The connector portion may comprise a tabconnector portion and/or a ball grid array connector portion. At leastone drive electrode may also be carried by the flexible layer.

The finger sensor may also include at least one electronic componentcarried by the flexible layer, such as a discrete component, a lightsource, a light detector, and another IC. The another IC may comprise atleast one other finger sensing IC, for example.

The IC finger sensor may comprise a semiconductor substrate having anupper surface. The finger sensing area may comprise an array of sensingelectrodes carried by the upper surface of the semiconductor substrate,such as for electric field finger sensing, for example.

A method aspect is for making a finger sensor. The method may includeproviding a finger sensing integrated circuit (IC) comprising a fingersensing area and at least one bond pad adjacent thereto, and positioninga flexible circuit adjacent the IC finger sensor. The flexible circuitmay include a flexible layer, at least one Electrostatic Discharge (ESD)electrode carried by the flexible layer, and at least one conductivetrace carried by the flexible layer. The method may further includecoupling at least one conductive trace to the at least one bond pad.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of a cellular telephone including afinger sensor in accordance with the invention.

FIG. 2 is an enlarged perspective view of a portion of the finger sensorshown in FIG. 1.

FIG. 3 is a plan view of a portion of the finger sensor as shown in FIG.1 with alternative embodiments of connector portions being illustrated.

FIG. 4 is an enlarged schematic cross-sectional view through a portionof the finger sensor as shown in FIG. 1.

FIG. 5 is a plan view of a portion of a finger sensor in accordance withthe invention, similar to FIG. 3, but showing a different embodiment ofa connector portion.

FIG. 6 is a schematic cross-sectional view of a mounted finger sensor inaccordance with the invention.

FIG. 7 is a schematic cross-sectional view of another embodiment of amounted finger sensor in accordance with the invention.

FIG. 8 is a schematic cross-sectional view of yet another embodiment ofa mounted finger sensor in accordance with the invention.

FIG. 9 is a schematic diagram illustrating some of the manufacturingsteps for a finger sensor as shown in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout and prime notation is used toindicate similar elements in alternative embodiments.

Referring initially to FIGS. 1-4, embodiments of a finger sensor 30 inaccordance with the invention are now described. The finger sensor 30 isillustratively mounted on an exposed surface of a candy bar-typecellular telephone 20. The illustrated candy bar-type cellular telephone20 is relatively compact and does not include a flip cover or otherarrangement to protect the finger sensor 30 as may be done in othertypes of cellular phones. Of course, the finger sensor 30 can also beused with these other more protective types of cell phones as will beappreciated by those skilled in the art. The finger sensor 30 can alsobe used with other portable and stationary electronic devices as well.The increased durability and ruggedness of the finger sensor 30 willpermit its widespread use even when exposed.

The cellular phone 20 includes a housing 21, a display 22 carried by thehousing, and processor/drive circuitry 23 also carried by the housingand connected to the display and to the finger sensor 30. An array ofinput keys 24 are also illustrated provided and used for conventionalcellphone dialing and other applications as will be appreciated by thoseskilled in the art. The processor/drive circuitry 23 also illustrativelyincludes a micro step-up transformer 25 that may be used in certainembodiments to increase the drive voltage for the finger sensor 30 asexplained in greater detail below.

The finger sensor 30 may of the slide type where the user's finger 26slides over the sensing area to generate a sequence of finger images.Alternatively, the finger sensor 30 could be of the static placementtype, where the user simply places his finger 26 onto the sensingsurface to generate a finger image. Of course, the finger sensor 30 mayalso include circuitry embedded therein and/or in cooperation with theprocessor/drive circuit 23 to provide menu navigation and selectionfunctions as will be appreciated by those skilled in the art.

As shown perhaps best in FIGS. 2 and 3, the finger sensor 30illustratively comprises a finger sensing integrated circuit (IC) 32including a finger sensing area 33 and a plurality of bond pads 34adjacent thereto. In particular, the finger sensing IC 32 may comprise asemiconductor substrate having an upper surface, and the finger sensingarea 33 may comprise an array of sensing electrodes carried by the uppersurface of the semiconductor substrate, such as for electric fieldfinger sensing, for example. Capacitive and/or thermal sensing pixelsmay also be used, for example.

The finger sensor 30 also includes a flexible circuit 35 coupled to theIC finger sensor. More particularly, the flexible circuit 35 includes aflexible layer 36 covering both the finger sensing area 33 and the bondpads 34 of the IC finger sensor 32. The flexible circuit 32 alsoincludes conductive traces 37 carried by the flexible layer 36 andcoupled to the bond pads 34. Of course, the flexible layer 36 preferablycomprises a material or combination of materials to permit fingersensing therethrough. Kapton is one such suitable material, althoughthose of skill in the art will readily recognize other suitablematerials. Kapton is also hydrophobic providing an advantage that it maypermit reading of partially wet or sweating fingers more readily, as anymoisture may tend to resist smearing across the image as will beappreciated by those skilled in the art.

As shown perhaps best in FIG. 3, the flexible circuit may comprise oneor more connector portions extending beyond the finger sensing area 33and the bond pads 34. As shown, for example, in the left hand portion ofFIG. 3, the connector portion may comprise a tab connector portion 40wherein the conductive traces 37 terminate at enlarged width portions ortabs 41. With reference to the right hand side of FIG. 3, an alternativeor additional connector portion may comprise the illustrated ball gridarray connector portion 42, wherein the conductive traces 37 areterminated at bumps or balls 43 as will be appreciated by those of skillin the art.

In the illustrated embodiment, the finger sensor 30 further includes anIC carrier 45 having a cavity receiving the finger sensing IC 32 therein(FIG. 4). The term IC carrier is meant to include any type of substrateor backing material on which or in which the finger sensing IC 32 ismounted. A fill material 46, such as an epoxy, is also illustrativelyprovided between the IC finger sensor 32 and the flexible circuit 35.Accordingly, the IC finger sensor 32 may be readily coupled to externalcircuitry, and may also enjoy enhanced robustness to potentialmechanical damage by finger or other object contact to the sensing areaof the IC finger sensor.

The sensor 30 also includes a pair of drive electrodes 50 carried on anouter and/or inner surface of the flexible layer 36 as seen perhaps bestin FIGS. 2 and 3. The drive electrodes 50 may be formed of the sameconductive material as the conductive traces 37 used for the connectorportions 40 or 42 as will also be appreciated by those skilled in theart. In other embodiments, only a single drive electrode 50 or more thantwo drive electrodes may be used. Even if the drive electrodes 50 arepositioned on the inner surface of the flexible layer 36 they can stillbe driven with a sufficient signal strength to operate. Thevoltage-boosting micro transformer 25 as shown in FIG. 1, may be used,for example, to achieve the desired drive voltage on the driveelectrodes 50 which may be up to about twenty volts for someembodiments.

The finger sensor 30 also includes one or more electrostatic discharge(ESD) electrodes 53 illustratively carried on the outer surface of theflexible layer 36 of the flexible circuit 35. Again the ESD electrodes53 may be formed of a conductive material applied or deposited onto theflexible layer 36 similar to the conductive traces 37 as will beappreciated by those skilled in the art. The ESD electrodes 53 may beconnected to a device ground, not shown, via one or more of theconductive traces 37.

As shown in the illustrated embodiment, the IC carrier 45 has agenerally rectangular shape with four beveled upper edges 55 as perhapsbest shown in FIG. 2. The beveled edges 55 are underlying or adjacentthe ESD electrode 53. Of course, in other embodiments, a differentnumber or only a single beveled edge 55 and adjacent ESD electrode 53may be used.

Referring now briefly to FIG. 5, another embodiment of flexible circuit35′ suitable for the finger sensor 30 is described. In this embodiment,the tab connector portion 40′ extends from the side of the flexiblelayer 36′ rather from an end as shown in FIG. 3. For clarity ofillustration, the right hand portion of the flexible layer 36 is notshown. Those other elements of FIG. 5 not specifically mentioned aresimilar to those corresponding elements described above with referenceto FIG. 3 and need no further discussion herein.

Referring now additionally to FIG. 6 mounting of the finger sensor 30 isnow described. In the illustrated embodiment, portions of the housingdefine an integral frame 21 surrounding the upper perimeter of theflexible circuit 35 that, in turn, is carried by the IC carrier 45. Thispositions the ESD electrodes 53 on the beveled edges of the IC carrier45. Moreover, the integral frame 21 has inclined surfaces correspondingto the beveled edges of the IC carrier 45. This defines ESD passages 63to the ESD electrodes 53 as will be appreciated by those skilled in theart. In other words, this packaging configuration will effectively drainoff ESD through a small gap 63 between the frame and the flexible layer36 and without having the ESD electrodes 53 directly exposed on theupper surface of the sensor 30.

The finger sensor 30 may further include at least one electroniccomponent 64 carried by the flexible layer as also explained withreference to FIG. 6. For example, the at least one electronic component64 may comprise at least one of a discrete component, a light source, alight detector, and another IC. If a light source or light detector isused, it will more likely be positioned so as to be on the upper surfaceof the sensor. U.S. Published Application No. 2005/0069180, assigned tothe assignee of the present invention and the entire contents of whichare incorporated herein by reference, discloses various infrared andoptical sensors and sources that may be used in combination with thepackaging features disclosed herein. Similarly, if another IC comprisesanother finger sensing IC, for example, it would also be positionedadjacent the IC 32 on the upper surface of the IC carrier 45 as will beappreciated by those skilled in the art. For example, two or more suchICs could be positioned so that their sensing areas were able to captureimages end-to-end, even if the chips themselves were staggered.Processing circuitry would stitch the images together widthwise in thisexample.

The mounting arrangement of FIG. 6 also illustrates another packagingaspect wherein a biasing member in the form of a body of resilientmaterial 62, such as foam, is positioned between the illustrated devicecircuit board 60 and the IC carrier 45. The resilient body of material62 permits the finger sensor 30 to be displaced downwardly or into thedevice to absorb shocks or blows, and causes the sensor to beresiliently pushed back into the desired alignment. The inclinedsurfaces of the integral frame and beveled edges 55 of the IC carrier 45also direct the proper alignment of the sensor 30 as it is restored toits upper position as will be appreciated by those skilled in the art.

A slightly different mounting arrangement for the finger sensor 30′ isexplained with additional reference to FIG. 7, wherein a separate frame21′ is provided that abuts adjacent housing portions 29′. Theillustrated frame 21′ also sets the finger sensing IC 32′ below thelevel of the adjacent housing portions 29′ for additional protection.Also, the biasing member is illustratively in the form of a backingplate 62′ that is not attached on all sides and is therefore free togive and provide a returning spring force as will be appreciated bythose skilled in the art. The backing plate may carry circuit traces tothereby serves as a circuit board as will be appreciated by thoseskilled in the art. Those other elements of FIG. 7 are similar to thoseindicated and described with reference to FIG. 6 and require no furtherdiscussion herein.

Yet another embodiment of a finger sensor 30″ is now described withreference to FIG. 8. In this embodiment, adjacent housing portionsdefine a frame 21″, along one or more sides of the IC carrier 45″. Theframe 21″ includes an upper portion 69″ and a downwardly extending guideportion 66″ offset from the upper portion that defines an interior stepor shoulder 67″. This step or shoulder 67″, in turn, cooperates with theIC carrier lateral projection or tab 68″ to define an upward stoparrangement. This tab 68″ may be integrally formed with the IC carrier45″ or comprise a separate piece connected to the main portion of thecarrier as will be appreciated by those skilled in the art. Accordingly,the IC carrier 45″ may be deflected downwardly, and will be biased backupwardly into its desired operating position along the guide portion66″.

The left hand portion of FIG. 8 shows an embodiment wherein the upwardstop arrangement is not provided along one side to thereby readilyaccommodate passage of the connector portion 40″. In yet otherembodiments, slots could be provided in the flexible circuit 35″ toaccommodate tabs 68″ to project therethrough and provide the upward stoparrangement as well. Those of skill in the art will appreciate otherconfigurations of stop arrangements and mounting.

Referring now additionally to FIG. 9, a method sequence for making thefinger sensor 30 is now described. Beginning at the top of the figure,the finger sensing IC 32 is flipped over and coupled to the flexiblecircuit 35 such as using an epoxy or other suitable fill material 46.Thereafter, as shown in the middle of the figure, the IC carrier 45 isadded to the assembly which is then illustratively rotated in the upwardfacing position. Lastly as shown in the lowermost portion of FIG. 8, thefinger sensor 30 is mounted between the frame 21 and the underlyingcircuit board 60. If the ball grid array connector portion 42 (FIG. 3)is used, this portion can be wrapped and secured underneath the ICcarrier 45 as will be readily appreciated by those skilled in the art.This is but one possible assembly sequence, and those of skill in theart will appreciate other similar assembly sequences as well.

The epoxy or glue 46 may be Z-axis conductive glue, and/or it mayincorporate resilient energy absorbing properties. The use of ananisotropic conductive material may physically extend the pixel'seffective electrical interface away from the die. The conductivematerial may contact the finger interface itself or it may terminate onthe underside of a top protective layer of material over the sensingarray. The same anisotropic conductive material may be used toelectrically bond the chip's external interface bond pads 34 toconductive traces 37 on the flexible layer 36.

The IC carrier 45 may be a plastic molding or other protective material,that may have resilient energy absorbing properties. It may incorporatemultiple layers of different materials, or graded materials having agradient in one or more physical properties such as stiffness. A stiff(non-stretching) but flexible material layer 36 (like Kapton) over asofter resilient material 46, all on top of the chip's surface 32,spreads the energy of a point impact across a larger area of the chipsurface. The resilient material to connect the chip to the circuit boardallows the chip—when under force—to move slightly with respect to thecircuit board, reducing the stress on the chip. The beveled mechanicalinterface between the IC carrier 45 and the frame 21 allows movement inboth the normal and shear directions with respect to relieve stress. Theflexible circuit 35 may also include conductive patterns or traces, notshown, in the area over the sensing array to enhance the RE imagingcapability.

The epoxy or glue 46 is a soft resilient layer between the stifferflexible layer 36 and the very stiff silicon chip surface. This allowsthe flexible layer 36 to bend inward to reduce scratching from sharppoints, and also reduce the transfer of sharp point forces to thesilicon.

The IC carrier 45 and any biasing member 62 provide mechanical supportto the silicon chip to prevent it from cracking when under stress, andmay seal the finger sensing IC 32 and its edges from the environment.The biasing member 62 between the IC carrier 45 and the circuit board 60can absorb shock energy in both the vertical and shear directions.

The top surface of a semiconductor chip is typically made of multiplelayers of brittle silicon oxides and soft aluminum. This type ofstructure may be easily scratched, cracked, and otherwise damaged whenforce is applied to a small point on that surface. Damage typicallyoccurs when the pressure applied to the insulating surface oxidepropagates through to the aluminum interconnect material directlybeneath it. The aluminum deforms removing support from under the oxide,which then bends and cracks. If sufficient force is applied this processmay continue through several alternating layers of silicon oxide andaluminum, short-circuiting the aluminum interconnects and degrading thechip's functionality.

In the package embodiments described herein, a sharp object approachingthe sensor first contacts the substrate layer (typically Kapton tape).The substrate material deforms and presses into the resilient gluematerial, spreading the force over a larger area and reducing themaximum force per area transmitted. The spread and diluted forcetransmitted through the resilient glue now causes the chip to movedownward away from the impacting object and into the resilient backingmaterial. Some of the impact energy is converting into motion of thechip and ultimately into compression of the resilient backing material.Finally, when the as chip is forced downward into the resilient backing,the chip will often tilt—encouraging the sharp object to deflect off thesensor. The stiffness of the various layers of resilient material areselected to protect the aluminum interconnects in the silicon chipagainst the most force possible.

The packaging concepts discussed above make a package that is: durableenough for use on the external surfaces of portable electronicequipment; and maintains good sensing signal propagation, resulting ingood quality sensor data. The embodiments are relatively inexpensive andstraightforward to manufacture in high volume.

Now reviewing a number of the possible advantages and features of thefinger sensors disclosed herein, significant improvements in scratchresistance can be achieved by combining a surface material like Kaptonthat is relatively stiff and difficult to tear, with a softer gluematerial underneath. With this structure, when a sharply pointed objectcomes into contact, the surface material can indent, reducing theinitial impact, spreading the force across a larger area, and preventingthe point from penetrating the surface. When the object is removed, theresilient materials return to their original shapes.

A flexible substrate with a smooth surface and a low coefficient offriction (such as a Kapton tape) will help resist abrasion. Theresilient structure described above can also improve abrasion resistanceby preventing the abrasive particles from cutting into the surface. Theresilient structure described above also provides several levels ofprotection against impacts of various intensities.

When a portable device like a cellphone is dropped, a shearing force isapplied to any structure that interconnects the case with the internalcircuit boards. In a sensor that is soldered to the internal circuitboard and projects through a hole in the case, the full shearing forceis applied to the sensor and its circuit board interconnects. In thepackage described above, the shear force is absorbed by the resilientmaterial that may mechanically connect the sensor to the circuit board.If the shear force is extreme, the beveled sensor will slip under thecase, converting the shear force into normal compression of theresilient backing material. When the impact event is over the sensorwill return to its normal position.

The package described can also provide protection against continuouspressure. When pressure is applied, the resilient backing compresses,allowing the sensor to retract from the surface a small distance. Inmany situations this will allow the case to carry more of the force,reducing the force on the sensor.

In the packaging described here, the flexible substrate material alsoacts as an ESD (electrostatic discharge) barrier between the chip andits environment, preventing ESD from reaching the sensitive electronicdevices on the chip. Accordingly, leakage current tingle may besignificantly reduced or eliminated. A 1 mil Kapton layer provides an8.6 Kv withstand capability. The ESD electrodes can capture dischargesat higher voltages. The maximum voltage over the drive electrodes priorto air breakdown to the ESD electrode is 7.5 Kv. The distance from thefarthest point of the drive electrode to the ESD capture electrodes is2.5 mm, and the dry air dielectric breakdown is 3 Kv/mm. Accordingly,even with a clean surface (worst case) the ESD would discharged to theESD capture electrode before penetrating the Kapton dielectric layer. Inaddition, over the array is provided 1 mil of Kaptom, plus 1 mil ofepoxy, plus 2.5 microns of SiN. This may provide about 14.1 Kvdielectric withstand over the pixel array. This may eliminate arequirement for outboard ESD suppressors and associated circuitry.

Some mechanical durability data is provided below in TABLE 1. Inparticular, three devices are compared: a model 1510 small slide IC witha nitride coating and no adhesive, a 1510 IC with a polyimide coatingand no adhesive, and a model 2501 large slide IC with a Kapton layer andacrylic adhesive/filler. The drill rod scratch and pencil scratch testsare ANSI tests. The other three tests are self-explanatory, and it canbe seen that the Kapton/filler device enjoys a considerable advantage interms of mechanical robustness.

TABLE 1 Bare 7 μm 25 μm Substrate Nitride Polyimide Kapton Adhesive N/AN/A Acrylic Test Die 1510 1510 2501 Ni Drill Rod Scratch (grams) <50 225350 Pencil Scratch (hardness) (5) N/A HB 6H 6.5 mm Ball Impact (gr cm)234 234 488 1.0 mm Ball Impact (gr cm) <75 <13 195 Rock Tumbler (hrs)N/A <8 67

All or part of the desired circuitry may be included and mounted on theflexible circuit. The customer interface cold then be a simple standardinterface, such as a USB connector interface. LEDs can be included onthe flexible circuit, or electroluminescent sources can be added asprinted films. Organic LEDs can be printed as films on the underside ofthe flexible circuit.

Other features and advantages in accordance with the invention may beunderstood with reference to copending applications entitled: FINGERSENSOR INCLUDING FLEXIBLE CIRCUIT AND ASSOCIATED METHODS, Ser. No.11/550,669, and FINGER SENSING WITH ENHANCED MOUNTING AND ASSOCIATEDMETHODS, Ser. No. 11/550,693 filed concurrently herewith and the entiredisclosures of which are incorporated herein by reference. Accordingly,many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that other modifications andembodiments are intended to be included within the scope of the appendedclaims.

1. A finger sensor comprising: a finger sensing integrated circuit (IC) comprising a finger sensing area and at least one bond pad adjacent thereto; a flexible circuit comprising a flexible layer covering said finger sensing area, and at least one conductive trace carried by said flexible layer and coupled to said at least one bond pad; a fill material between said finger sensing IC and said flexible circuit and covering said finger sensing area; said flexible layer permitting finger sensing therethrough; and at least one Electrostatic Discharge (ESD) electrode carried by said flexible layer.
 2. A finger sensor according to claim 1 further comprising an IC carrier having a cavity receiving said finger sensing IC therein.
 3. A finger sensor according to claim 2 further comprising a frame surrounding at least a portion of an upper perimeter of said flexible layer and defining therewith at least one ESD passage to said at least one ESD electrode.
 4. A finger sensor according to claim 1 wherein said flexible circuit comprises at least one connector portion extending beyond said finger sensing area and said at least one bond pad.
 5. A finger sensor according to claim 4 wherein said at least one connector portion comprises at least one tab connector portion.
 6. A finger sensor according to claim 4 wherein said at least one connector portion comprises at least one ball grid array connector portion.
 7. A finger sensor according to claim 1 further comprising at least one drive electrode carried by said flexible layer.
 8. A finger sensor according to claim 1 further comprising at least one electronic component carried by said flexible layer.
 9. A finger sensor according to claim 8 wherein said at least one electronic component comprises at least one of a discrete component, a light source, a light detector, and another IC.
 10. A finger sensor according to claim 9 wherein said another IC comprises at least one other finger sensing IC.
 11. A finger sensor according to claim 1 wherein said finger sensing IC comprises a semiconductor substrate having an upper surface; and wherein said finger sensing area comprises an array of sensing electrodes carried by the upper surface of said semiconductor substrate.
 12. A finger sensor comprising: a finger sensing integrated circuit (IC) comprising a finger sensing area and a plurality of bond pads adjacent thereto; a flexible circuit comprising a flexible layer, and a plurality of conductive traces carried thereby and coupled to said plurality of bond pads; a fill material between said finger sensing IC and said flexible circuit and covering said finger sensing area; at least one Electrostatic Discharge (ESD) electrode carried by said flexible layer; an IC carrier having a cavity receiving said finger sensing IC therein, said IC carrier having at least one beveled edge adjacent said at least one ESD electrode; and a frame surrounding at least a portion of an upper perimeter of said flexible layer and defining therewith at least one ESD passage to said at least one ESD electrode.
 13. A finger sensor according to claim 12 wherein said flexible circuit comprises at least one connector portion extending beyond said finger sensing area and said plurality of bond pads.
 14. A finger sensor according to claim 12 further comprising at least one drive electrode carried by said flexible layer.
 15. A finger sensor according to claim 12 further comprising at least one electronic component carried by said flexible layer.
 16. A finger sensor according to claim 12 wherein said finger sensing IC comprises a semiconductor substrate having an upper surface; and wherein said finger sensing area comprises an array of sensing electrodes carried by the upper surface of said semiconductor substrate.
 17. A method for making a finger sensor comprising: providing a finger sensing integrated circuit (IC) comprising a finger sensing area and at least one bond pad adjacent thereto; positioning a flexible circuit adjacent the IC finger sensor, the flexible circuit comprising a flexible layer, at least one Electrostatic Discharge (ESD) electrode carried by the flexible layer, and at least one conductive trace carried by the flexible layer; and coupling the at least one conductive trace to the at least one bond pad.
 18. A method according to claim 17 wherein the at least one ESD electrode is coupled to at least one conductive trace.
 19. A method according to claim 17 further comprising receiving the IC finger sensor in a cavity of an IC carrier.
 20. A method according to claim 19 further comprising positioning a frame to surround at least a portion of an upper perimeter of the flexible layer and define therewith at least one ESD passage to the at least one ESD electrode.
 21. A method according to claim 17 further comprising positioning a fill material between the IC finger sensor and the flexible circuit.
 22. A method according to claim 17 wherein the flexible circuit comprises at least one connector portion extending beyond the finger sensing area and the plurality of bond pads.
 23. A method according to claim 17 further comprising forming at least one drive electrode carried by the flexible layer.
 24. A method according to claim 17 further comprising positioning at least one electronic component carried by the flexible layer.
 25. A method according to claim 17 wherein the IC finger sensor comprises a semiconductor substrate having an upper surface; and wherein the finger sensing area comprises an array of sensing electrodes carried by the upper surface of the semiconductor substrate.
 26. A finger sensor comprising: a finger sensing integrated circuit (IC) comprising a finger sensing area and at least one bond pad adjacent thereto; a flexible circuit comprising a flexible layer covering said finger sensing area and having at least one connector portion extending beyond said finger sensing area and said at least one bond pad, and at least one conductive trace carried by said flexible layer and coupled to said at least one bond pad; said flexible layer permitting finger sensing therethrough; at least one electronic component carried by said connector portion extending beyond said finger sensing area and said at least one bond pad; and at least one Electrostatic Discharge (ESD) electrode carried by said flexible layer.
 27. A finger sensor according to claim 26 further comprising at least one drive electrode carried by said flexible layer.
 28. A finger sensor according to claim 26 wherein said at least one electronic component comprises at least one of a discrete component, a light source, a light detector, and another IC.
 29. A finger sensor according to claim 28 wherein said another IC comprises at least one other finger sensing IC.
 30. A finger sensor according to claim 26 wherein said finger sensing IC comprises a semiconductor substrate having an upper surface; and wherein said finger sensing area comprises an array of sensing electrodes carried by the upper surface of said semiconductor substrate. 